Laser isotope separation method employing isotopically selective collisional relaxation

ABSTRACT

A method of separating isotopes from polyatomic molecules, in particular  13 C from trifluoromethane HCF 3 , by applying to the polyatomic molecules in the gas phase two infrared laser beams of different frequencies. The first laser has a frequency appropriate to excite a low overtone transition of a light atom stretch vibration and produce vibrationally pre-excited molecules enriched in the desired isotopes, for instance  13 C. The second laser has a frequency and energy fluence to selectively induce dissociation of the vibrationally pre-excited molecules by infrared multiphoton excitation. The product of the pressure of the molecules and the time-delay of the second laser pulse relative to the first allows collisional vibrational deactivation of a substantial amount of the vibrationally pre-excited molecules containing non-desired isotope(s), like  12 C, before dissociation of the vibrationally excited molecules occurs, while there is no significant collisional vibrational deactivation of pre-excited molecules containing the desired isotope, like  13 C. The dissociation products are hence highly enriched in the desired isotope.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the national phase application of InternationalApplication No. PCT/IB00/00639 filed May 12, 2000, which claims priorityof EP No. 99810688.4 filed Jul. 29, 1999, entitled “Laser IsotopeSeparation Method Employing Isotopically Selective CollisionRelaxation.”

FIELD OF THE INVENTION

This invention relates to the separation of a desired isotope frompolyatomic molecules containing different isotopes, by applying to themolecules in the gas phase at a predetermined pressure, near infraredradiation of a first pulsed laser and, after a predetermined time-lagwhich allows a sufficient number of collisions, infrared radiation of asecond pulsed laser of different frequency to produce a chemicalreaction resulting in a molecule, enriched in the desired isotope, whichcan be separated from the remainder of the material. The invention isexemplified in a particular by the separation of ¹³C isotopes inpolaytomic molecules consisting of mostly ¹²C isotopes and which containC—H and C—F bonds.

BACKGROUND OF THE INVENTION

The stable ¹³C isotope has been widely used in many applications butuntil recently in relatively small volume. Recent medical development ofthe so-called carbon-13 Diagnostic Breath Test (¹³C DBT) (U.S. Pat. No.4,830,010) has dramatically changed the situation. The DBTs are used toassess the condition of organs of the human digestive system. Because ofits safety, relative simplicity and wide range of application, the DBTtechnology has rapidly increased the demand for ¹³C.

A limiting factor for the growth in the use of DBTs is the relativelyhigh production cost of highly (>99%) isotopically pure ¹³C. The bulk ofthe ¹³C at present is produced by multi-cycle low temperaturedistillation of CO. This technique is well developed and has nearlyreached the maximum of its efficiency, limited by its high energyconsumption.

The molecular laser isotope separation (MLIS) approach provides analternative for production of high purity stable isotopes. The mostdeveloped method for MLIS of ¹³C is based on infrared multiphotondissociation (IRMPD) of CF₂HCl by a pulsed CO₂ laser. This method relieson a 20 cm⁻¹ isotopic shift in the IR absorption spectrum of the ¹³Ccontaining molecules relative to ¹²C containing molecules for selectiveabsorption and dissociation of ¹³CF₂HCl. The CF₂ dissociation fragmentsrecombine, resulting in stable C₂F₄ molecules that are separated fromparent molecules by distillation.

An example of a recent implementation of this approach by Ivanenko etal. (Applied Physics B, 62, pp. 329-332, 1996) produce a macroscopicenrichment of ¹³C using a high-power high repetition rate industrial CO₂laser. A report by V. Y. Baranov et al., (Proceedings of 4th All-RussianInternational Scientific Conference on “Physical Chemical Processes atSelection of Atoms and Molecules”, 1999, pp. 12-16) describes a nearcompleted pilot plant in Kaliningrad, Russia, which is designed toproduce several tens of kilograms of isotopically pure ¹³C a year usingthe same approach. In both cases, CF₂HCl is enriched to 30-50% in ¹³C byselective IRMPD. In both cases, it is suggested that further enrichmentof the products up to 99% ¹³C could be accomplished by non-lasertechniques such as centrifugation. In another approach, a second stageof laser separation is employed to bring partially enriched product tohigher levels of enrichment (Ph. Ma et al., Appl. Phys. B 49 503(1989)). In the case of ¹³C isotope separation using CF₂HCl as astarting material, the partially enriched product (C₂F₄) is chemicallyconverted to a molecule suitable for the next laser isotope separationcycle (A. P. Dyad'kin, et al.; Proceedings of 4th All-RussianInternational Scientific Conference on “Physical Chemical Processes atSelection of Atoms and Molecules”, 1999, pp. 17-20). This extra stagecomplicates the overall process and significantly increases cost of theproduct.

Under certain conditions, single-laser IRMPD of CF₂HCl has demonstratedthe capability of producing products highly enriched in ¹³C in a singlestage, but this high degree of enrichment comes at the cost ofproductivity. The work of Gauthier et al. (Appl. Phys. B. 28, 2, 1982)achieves enrichment to 96%, but this requires operating at low laserfluence and low pressure, both of which decrease he productivity.Reasonable productivity is achieved at only 50% ¹³C enrichment, whichfalls short of the high purity (>99%) required for medical applications.

One approach to increase the selectivity in laser isotope separation isto use a single-stage two-laser process. U.S. Pat. No. 4,461,686 relatesto two-color IR—IR MLIS wherein a first laser excites a non-specifiedvibrational state and a second laser excites molecules up to a level ofa chemical conversion, including dissociation. A similar method has beensuccessfully realized on a laboratory scale by Evseev et al. (Appl.Phys. B36, 93, 1985; Sov. J. Quantum Electron. 18, 385, 1988). Whilethis approach overcomes some of the drawbacks of a single-laser processand achieves relatively high selectivity (S=6000 which corresponds to¹³C enriched to 98.5%), low pressure is still required, limiting theproductivity.

One widely known problem of two-laser isotope separation schemes is thepossibility of vibrational relaxation of the molecules in the timebetween the two laser pulses, leading to loss of isotopic selectivity,U.S. Pat. No. 4,461,686 clearly states this problem by specifying a timedelay between laser pulses that is shorter than the vibrationalrelaxation time but longer than the rotational relaxation time of thepolyatomic molecules, allowing time for rotational but not forvibrational relaxation.

A number of other two-laser schemes have been employed for separation ofvarious isotopic species, but in most cases, conditions are adjusted tominimize collisions in the time between the two laser pulses and/or thedeleterious effects of collisions on the selectivity is explicitlymentioned. In their two-color infrared isotopically selectivedecomposition of UF₆, Rabinowitz et al. (Optics Letters 7, 212 (1982))indicate that they use pressures of less than 10⁻⁷ Torr during runs,ensuring collision free reactions. They clearly state thatenergy-exchange collisions between the two isotopic species may scramblethe selectivity. Using a similar two-color laser isotope separationscheme for SeF₆, Tiee and Wittig (J. Chem. Phys. 69, 4756 (1978)), statethat they use a delay between the two lasers that is short enough sothat deleterious energy transfer processes do not have a chance tointerfere. In their two-color multiple photon dissociation of CF₃T,Pateopol and O-Neil (Laser Isotope Separation, SPIE, Vol. 1859, p.210-218 (1993)) show in FIG. 4 that an increase in pressure, whichincreases the frequency of collisions, decreases the isotopicselectivity. In a two laser scheme for separation of sulfur isotopes,French patent FR2530966A does not explicitly mention collisions but usessufficiently low pressure and short time delay such that vibrationalrelaxation from collisions between the two laser pulses is minimized. Intheir two laser dissociation scheme for OsO₄, Ambartzumian et al.(Optics Letters 1, 22 (1977)) do not mention collisions, however theinformation they provide on the experimental conditions, particularlythe low pressure (˜0.3 Torr) suggests that no collisional vibrationalrelaxation occurs during the process.

A few studies have observed that under certain conditions, collisionsseem to enhance the isotopic selectivity. In their single-laser IRMPD ofCF₂HCl for ¹³C enrichment, Gauthier et al. (Appl. Phys. B. 28, 2, 1982,FIG. 3) demonstrate increasing selectivity with increasing pressure.This increase in selectivity is accompanied with a correspondingdecrease in dissociation efficiency (also FIG. 3), leading to low valuesof productivity. In their two-laser IRMPD studies of CF₂HCl for ¹³Cenrichment, Evseev et al. (Appl. Phys. B36, 93, 1985; Sov. J. QuantumElectron. 18, 385, 1988) observe modest increase in selectivity bothupon increase in the pressure of the working gas as well as uponincreasing the delay between the two lasers. They attribute theincreased selectivity to different rates of vibrational—vibrationalexchange of “hot” ensembles of ¹²C and ¹³C containing molecules with theensemble of “cold” unexcited molecules of the main isotope, althoughthey propose no explanation for the rate difference.

We believe that the attribution by Evseev et al. of the pressure andtime-delay dependence of the isotopic selectivity to a difference incollisional deactivation rates is essentially correct, although theparticular pre-excitation technique that they use, namely CO₂ laserinfrared multiphoton excitation (IRMPE), prohibits them from exploitingthis effect for simultaneously achieving both high selectivity and highproductivity in ¹³C isotope separation. IRMPE can either pre-excitemolecules to a few low energy vibrational levels when the laser fluenceis low, or to a wider distribution of higher energy levels if the laserfluence is high. In both cases, the collisional effect can provide onlya limited improvement of selectivity. Indeed, the 6000 maximum isotopicselectivity in their work has been achieved only for relatively lowpressure (2.5 Torr) and only for cold molecules (−65° C.). Coolingmolecule to temperatures in the range of −60 to −70° C. itself typicallyincreases selectivity of this process by a few times.

The process that is the subject of this present invention makes use ofour fundamental understanding of the mechanism of isotopically selectivecollisional vibrational relaxation to devise a two-laser isotopeseparation scheme that can make optimal use of this collisionalphenomenon. Our experiments show that a selectivity of greater than 9000can be achieved at room temperature and at pressures greater than 50Torr.

SUMMARY OF THE INVENTION

An object of the invention is to provide a two-laser infraredmultiphoton dissociation process for isotope separation that can producehighly isotopically enriched species in a single stage.

According to the invention, this object is achieved by the method as setout below.

In this method, the radiation of the first laser has a predeterminedfrequency to excite by a single-photon a low overtone vibrationaltransition of the polyatomic parent molecules, in particular a hydrogenstretch vibration, to produce vibrational pre-excited molecules at awell defined energy enriched in the desired isotope, for instance ¹³C.

The radiation of the second laser has a predetermined frequency andpredetermined energy fluence to induce selective dissociation of thevibrationally pre-excited excited molecules by infrared multiphotonexcitation, in particular of a C-F stretch vibration.

The product of the pressure of the molecules and the time-lag Δt betweenthe pre-excitation by the first laser pulse and the dissociation duringthe second laser pulse (which results from the effective length of thesecond laser pulse plus any time delay of the second laser pulserelative to the first laser pulse), is sufficiently high to allowcollisional vibrational deactivation of a substantial amount of thevibrationally pre-excited molecules containing non-desired isotope(s),like ¹²C, before dissociation of the vibrationally excited moleculesoccurs while having no significant collisional vibrational deactivationof the pre-excited molecules containing the desired isotope, like ¹³C.The dissociation products are hence more highly enriched in the desiredisotope as a result of collisions.

Collisions that occur between the two laser pulses and/or during thesecond pulse are hence used to increase significantly the isotopicselectivity.

As is described more fully below, the use of collisions to significantlyincrease the isotopic selectivity requires excitation by the first laserto a well defined energy of at least several thousand cm⁻¹. This isaccomplished by direct, single photon excitation of a low overtone (Δv=2or 3) of a hydrogen atom stretch vibration. The combination oflow-overtone excitation by the first laser with isotopically selectivecollisional deactivation in the time between two laser pulses, followedby selective IRMPD of the pre-excited molecules induced by the secondlaser represents a unique feature of this invention.

This approach has several important advantages over otherimplementations of other IRMPD isotope separation schemes. First,vibrational overtone excitation with a continuously tunable laser canreach the maximum selectivity determined by the overlap in the spectraof two isotopic species, while conventional line-tunable CO₂ laserscannot be sure to hit the point of minimum spectral overlap. Moreover,isotope shifts are in general greater for overtone transitions than forvibrational fundamentals. Secondly, overtone pre-excitation of a lightatom stretch vibration can promote molecules directly to the vibrationalquasicontinuum with a well defined energy, allowing the parameters ofthe dissociating laser to be optimized for this energy, preserving theisotopic selectivity gained in the first step. Because the IRMPDprocesses is applied to molecules already in the vibrationalquasicontinuum, efficient dissociation occurs at relatively low (0.5-3J/cm²) CO₂ laser fluence, avoiding the need to focus the CO₂ laser beam.This permits a great increase of the irradiated volume, since collimatedbeams can be propagated together for meters, limited only by the beamdivergence. While this is an important feature, it is not unique to ourprocess. Most importantly, using vibrational overtone excitation for thefirst step, followed by collisions of the pre-excited molecules,enhances the isotopic selectivity of the process and at the same timeallows higher working pressures where the density of molecules ishigher, leading simultaneously to high selectivity (>99% isotopicpurity) and reasonable productivity in a single stage process. This canmake the process economically feasible and competitive with the currenttechnologies.

Taken together, these factors indicate that the overtoneexcitation-IRMPD scheme according to the invention should provide a moreefficient and selective means of laser isotope separation thanpreviously developed MLIS schemes. The results described belowdemonstrate that this is indeed the case.

As mentioned above, isotope separation can operate with low fluencelaser beams enabling interaction by multiphoton dissociation over alarge volume. Consequently, the first and second laser beams can becollimated or slightly diverging or slightly converging beams of lowfluence (≦5 J/cm²) overlapping with one another over a substantialportion or all of their respective volumes containing the saidpolyatomic molecules. The first and second beams can have an angle ofdivergence/convergence less than 2.0×10⁻³ rad.

The method according to the invention is particularly advantageous forseparating ¹³C isotopes from polyatomic molecules consisting of mostly¹²C isotopes and which contain C—H and C—F bonds, for example moleculesof the formula HCF₂X, wherein X is F, Cl, B or I. There have been anumber of other papers and patents that share these working moleculesbut use a completely different process which does not use collisions toenhance selectivity. We do not claim this class of compounds for laserisotope separation in general, but only as suitable candidates under thespecific conditions of our process.

In one example, the molecules are trifluoromethane HCF₃, the frequencyof the first laser is 8753±1 cm⁻¹ or 8549±1 cm⁻¹, the frequency of thesecond laser is in the range 1020-1070 cm⁻¹, and the predeterminedenergy fluence of the second laser has a value in the range 0.5-5 J/cm²depending on the pulse shape of the second laser. Alternatively, fortrifluoromethane HCF₃, the frequency of the first laser is 5936.5±1 cm⁻¹or 5681±1 cm⁻¹.

Alternatively, the molecules are CF₂HCl and the predetermined frequencyof the first laser is 5911±5 cm⁻¹ or 8693±2 cm⁻¹, or the molecules aremonofluoromethane CH₃F.

The first predetermined frequency can be produced by stimulated Ramanscattering of narrowband tunable radiation of a solid state pulsed laserand the second predetermined frequency is produced by a pulsed CO₂laser.

The method of the invention can also be applied to the separation ofisotopes from other molecules including SiH₄, SiF₃H, SiCl₃H, GeH₄, andalcohols of the formula R—OH, where R=CH₃, C₂H₅, C₃H₇ or C₄H₉.

The overlapping first and second laser beams can be substantiallyparallel or can multiple intersect.

BRIEF DESCRIPTION OF DRAWINGS

In the drawings, given by way of example:

FIG. 1 is an energy level schematic for the isotope separation methodaccording to the invention;

FIG. 2 is a diagram of an apparatus used to carry out the isotopeseparation method according to the invention;

FIG. 3 shows a typical mass-spectrum of C₂F₄ products enriched with ¹³Cisotopes by the method according to the invention;

FIG. 4 shows the concentration of ¹²C and ¹³C in the C₂F₄ dissociationproduct obtained in an experimental set-up, as a function of thepressure at zero time-delay between the two pulses (time-lag Δt≈35 ns);and

FIG. 5 shows the concentration of ¹²C and ¹³C in the C₂F₄ dissociationproduct and the percentage of ¹³C in the C₂F₄ dissociation product as afunction of time-delay between the overtone excitation laser pulse andthe CO₂ laser pulse.

DETAILED DESCRIPTION

This description uses the example of CF₃H as a working molecule for ¹³Cisotope separation, although the applicability of the invention isbroader in terms of both potential working molecules as well as isotopesthat can be separated. The method according to the invention comprisestwo steps, as shown schematically in FIG. 1. In the first step, anear-infrared laser pulse pre-excites molecules containing primarily thedesired isotope via a low (Δv=2 or 3) overtone transition of the CHstretch vibration. Following this, a CO₂ laser pulse excites a C-Fstretch of only the pre-excited molecules, selectively dissociating themby IRMPD, producing CF₂+HF. The isotopically selected CF₂ radicals arecollected after combining to form C₂F₄.

Using a narrow bandwidth, continuously tunable laser for pre-excitation,we can choose the excitation frequency to be in exact resonance with anovertone transition of the desired isotopic species, provided therotational structure of the overtone band is at least partiallyresolved. Even if the isotopic spectral shift is relatively small, givensufficient resolution it is possible to find parts of the spectrum wherethere is a minimum of overlap between the different isotopic species. Inthe case of ¹³C isotope separation using CF₃H as a parent molecule, thispre-excitation usually produces at least 80% ¹³C isotopes and at most20% ¹²C isotopes, possibly about 90% ¹³C isotopes and about 10% ¹²Cisotopes.

The second laser beam, the role of which is to dissociate pre-excitedmolecule via IRMPD, is arranged with a pulse length and/or delayrelative to the first laser beam sufficient to allow a substantialamount of vibrationally excited molecules containing ¹²C to relax byvibrational state changing collisions to an energy from which they willnot be efficiently dissociated. During this time, a substantial amountof the molecules containing ¹³C remain excited and available to bedissociated by the second laser pulse. Achieving enrichment to >99% in¹³C isotopes in a single pass requires a substantial difference in thecollisional vibrational relaxation rates of pre-excited molecules ofdifferent isotopic composition. It is not evident from the publishedliterature that the vibrational relaxation rates of highly vibrationallyexcited molecules should show a strong isotope dependence. Previousstudies of diatomic and triatomic molecules have shown that the detuningof energy levels upon isotopic substitution is sufficient to change theenergy gap and hence the energy transfer rate in molecules excited withone quantum of vibrational excitation. For example, Stephenson et al.(J. Chem. Phys. 48, 4790 (1968)) have shown that for the following mixedisotope collisions in CO₂,

¹²C¹⁶O¹⁶O (v₃=1)+¹²C¹⁶O¹⁸O (v₃=0)→¹²C¹⁶O¹⁶O (v₃=0)+¹²C¹⁶O¹⁸O (v₃=1)

ΔE=18 cm⁻¹

¹²C¹⁶O¹⁶O (v₃=1)+¹³C¹⁶O¹⁶O (v₃=1)→¹²C¹⁶O¹⁶O (v₃=0)+¹³C¹⁶O¹⁶O (v₃=1)

ΔE=66 cm⁻¹

the more than three-fold difference in the energy gap results in adifference in vibrational deactivation rate of a factor of 3.

One cannot expect a similar fractional difference in the energy gap tooccur in a polyatomic molecule excited to a vibrational overtone level,however. For example, consider the specific case of a CF₃H moleculeprepared in the v_(CH)=3 level prior to collision with other CF₃Hmolecules that are vibrationally unexcited. For the ¹³C speciescolliding with unexcited ¹²C molecules, the energetics of this processwould be given by

¹³CF₃H(v_(CH)=3)+¹²CF₃H(v_(CH)=0)→¹³CF₃H(v_(CH)=2)+¹²CF₃H(v_(CH)=1)

ΔE=−254 cm⁻¹

while for vibrationally excited ¹²C molecules colliding with unexcited¹²C molecules one would have

¹²CF₃H(v_(CH)=3)+¹²CF₃H(v_(CH)=0)→¹²CF₃H(v_(CH)=2)+¹²CF₃H(v_(CH)=1)

ΔE=−240 cm⁻¹

The difference in the energy gap of these two processes comes from theisotope effect on the difference in the v=3 to v=2 energy spacing. Forboth isotopic species, the 3→2 de-excitation process will be a fewhundred cm⁻¹ out of resonance with the 0→1 excitation process because ofthe large anharmonicity of the C-H stretch. The small fractionaldifference in the energy deficit of the process between the ¹³C and ¹²Cspecies is significantly less than the simple case of CO₂ shown above.It is difficult to imagine that such a small fractional difference inenergy gap could lead to a substantial difference in vibrational energytransfer rates in CF₃H.

The key to understanding the phenomenon of collisional enhanced isotopeselectivity exploited in this invention is to realize that the initiallyprepared state of CF₃H is not a pure v_(CH)=3 stretch state. Vibrationalstate-mixing in these molecules has been extensively studied and is wellunderstood (J. Segall et al., J. Chem. Phys. 86, 634 (1987)). Eachovertone level with N CH stretch quanta is characterized by a polyad ofN+1 strongly coupled states consisting of combinations of stretch andbend excitations. Thus for N=3, one has the group of coupled states(designated |v_(s), v_(b)> for the number of stretch and bend quantarespectively) |3,0>, |2,2>, |1,4>, and |0,6>. The eigenstates in thisenergy region can be expressed as linear combinations of thesezeroth-order states:

|j>=c₁|3,0>+c₂|2,2>+c₃ |1,4>+c ₄|0,6>

Higher resolution spectroscopy of the CH stretch overtone levels in thismolecule reveals that these “first-order” states are weakly coupled toother nearby states (O. V. Boyarkin and T. R. Rizzo, J. Chem. Phys. 105,6285 (1996)). In the limit that all the available states within a smallenergy region about the v_(CH)=3 level are coupled in some measure, astate in this energy region can be represented as the following mixture${j\rangle} = {{c_{3}{3\rangle}{0\rangle}\quad {0\rangle}\quad \ldots \quad {0\rangle}} + {{2\rangle}\quad {\sum\limits_{i}^{\quad}\quad {c_{2i}\quad {\prod\limits_{k = 2}^{9}\quad {v_{ki}\rangle}}}}} + {{1\rangle}{\sum\limits_{i}^{\quad}\quad {c_{1i}\quad {\prod\limits_{k = 2}^{9}\quad {v_{ki}\rangle}}}}} + {{0\rangle}{\sum\limits_{i}^{\quad}\quad {c_{0i}\quad {\prod\limits_{k = 2}^{9}{v_{ki}\rangle}}}}}}$

where the zeroth-order states are grouped by their number of CH stretchquanta. A simple state count would reveal that the most numerouszeroth-order states in this mixture are those with 0 quanta of CHstretch mode. In fact, the average occupation number of all thedifferent vibrational modes of such a mixed state with ˜9000 cm⁻¹ ofenergy is between 0 and 1.

Let us now consider the effect of state-mixing on the collisional energytransfer process. In a sense, one can expect this mixed state to behavemore like a molecule with one quantum of vibration. In this case, therelevant processes for us to consider are now

¹³CF₃H (v=1)+¹²CF₃H (v=0)→¹³CF₃H (v=0)+¹²CF₃H (v=1)

and

¹²CF₃H (v=1)+¹²CF₃H (v=0)→¹²CF₃H (v=0)+¹²CF₃H (v=1)

where the vibrational quantum number v could refer to any vibrationalmode. This process will be more nearly resonant as in the case of CO₂presented above. For collisions between ¹²C containing molecules, theonly energy deficit will be due to the cumulative effect of theoff-diagonal anharmonicities between the C-H stretch mode and the othermodes of the molecule. For collisions between ¹³C and ¹²C containingmolecules, the energy deficit will be the sum of the these off-diagonalanharmonicities together with the isotope shifts. For every mode, theisotope shift will make the collisions of ¹³C containing molecules befurther off-resonant than the ¹²C containing molecules. Since mostoff-diagonal anharmonicities tend to be small, the isotope shift shouldrepresent a substantial fraction of the energy gap for the energytransfer process. In view of the steep dependence of the v—v transferrate on the energy gap, this would lead to a substantial difference inrelaxation rates for vibrationally excited ¹³CF₃H and ¹²CF₃H.

This model for the detailed mechanism of the difference in collisionalrelaxation rates between two isotopomers allow us to chose conditions inwhich we can fully exploit this phenomenon for isotope separation. Inorder to use collisional enhancement of the isotopic selectivityeffectively, the first laser excitation step needs to fulfil severalrequirements:

(1) It must pre-excite molecules to a sufficiently high energy such thatthe vibrational states are substantially mixed in the sense describedabove. All molecules exhibit such state-mixing, but the energy at whichit occurs is molecule specific.

(2) It must pre-excite molecules to a sufficiently high energy such thatthe second laser can selectively dissociate pre-excited molecules andnot unexcited molecules.

(3) It must pre-excite molecules to a well defined energy, such that theparameters of the second laser can be optimized to dissociateselectively those molecules that have not undergone substantialvibration energy relaxation by collisions. This requires single-photonand not multiple-photon excitation.

If condition (1) is not fulfilled, the collisional vibrationalrelaxation rates of the two isotopes of a highly excited molecule wouldnot be sufficiently different to enhance the selectivity of the isotopeseparation process. If either condition (2) or condition (3) is notfulfilled, the second laser will not be able to selectively dissociatethe desired isotope, even if the vibrational relaxation rates aredifferent. Previous two-laser isotope separation techniques do notfulfill these requirements and hence are not able to achieve both highselectivity and reasonable productivity simultaneously.

The following Example illustrates experiments underlying the inventionand its implementation on laboratory scale.

EXAMPLE

Exemplary of this new approach for isotope separation, trifluoromethane,CF₃H, is used as the parent molecule for ¹³C isotope separation.Trifluoromethane was chosen for the following reasons:

(1) It has an IR active light atom vibration associated with the carbonatom (¹³C-H stretch, υ₁=3025.3 cm⁻¹), through which a substantial amountof vibrational energy can be deposited into a molecule via a lowovertone transition. The isotopic shift in the 3υ₁ band of 39.7 cm⁻¹ isappreciable.

(2) The vibrational states accessed by a CH stretch overtone excitationare substantially mixed.

(3) It has another IR active vibration with a fundamental frequencyshifted to the high frequency side of a conventional CO₂ laser (¹³C-Fstretch, υ₅=1132.4 cm⁻¹). One can expect that the optimum frequency forIRMPD of vibrationally excited molecules lies within tuning range of CO₂laser.

(4) The IRLAPS detection technique has been successfully implemented forstudying the overtone spectroscopy of this molecule, and we know thatIRMPD of vibrationally excited CF₃H can be fulfilled with highselectivity. See for example papers by Boyarkin/Settle/Rizzo in Ber.Bunseges. Phys. Chem. 99, 504 (1995) and by Boyarkin/Rizzo in J. Chem.Phys. 105, 6285 (1996).

(5) The lowest dissociation channel for CF₃H produces CF₂ and HF.Because the CF₂ fragment is the same as that from the IRMPD of CF₂HCl,we can take advantage of the large body of work on CF₂ collection inthis highly studied system.

FIG. 2 schematically illustrates an experimental apparatus, comprising acylindrical glass cell 1 (which is shown in two-dimensional projection,2 cm in diameter, 50 cm long) with BaF₂ windows filled with CF₃H to aspecific pressure as measured by a capacitance manometer.

A 20-25 mJ pre-excitation laser pulse 2, generated by Raman shifting a90 mJ pulse from a Nd:YAG pumped dye laser (Spectra Physics GCR-270,Lumonics HP 500) in high pressure H₂, promotes CF₃H molecules in cell 1at 7.5 mbar pressure to the 3υ₁ (CH stretch) level via the Q-branch at8753 cm⁻¹. This laser pulse 2, which has a duration of 5-6 nanoseconds,is focused into the center of cell 1 by an F=+120 cm lens 4, giving anestimated maximum fluence of 1-2 J/cm² at the beam waist.

After a fixed delay, a pulse 3 from the CO₂ laser (Lumonics, TEA-850)arrives and selectively dissociates the vibrationally pre-excitedmolecules via IRMPD to produce CF₂ and HF. This beam 3 is firsttruncated to a size of 5-10 mm diameter by passing it through anadjustable iris and then focused by an F=+75 cm lens 5 to a 2×2 mm² beamwaist. Its fluence can be varied over a wide range. This pulse consistsof a peak of 150 ns FWHM followed by a 2-3 μsec tail carrying more than60% of the total pulse energy. The two laser beams 2,3 (i.e. thepre-excitation laser and CO₂ laser) are combined on a 10 mm thick BaF₂Pellin-Broca prism 6 and enter the cell 1 from the same side.

The CF₂ dissociation products eventually recombine to form C₂F₄, whichis sampled from cell 1 at 7. The relative concentrations of the C₂F₄with different carbon isotopes are measured in quadrupolemass-spectrometer 8 at atomic masses 100 (¹²C₂F₄), 101 (¹²CF₂ ¹³CF₂) and102 (¹³C₂F₄). The productivity of the process for ¹³C is determined astwice the integral of the signal at mass 102 plus the integral of signalat mass 101. Correspondingly, the productivity of ¹²C is twice theintegral of the signal at mass 100 plus the integral of signal at mass101. The percentage of ¹³C is determined as a ratio of the ¹³Cproductivity to the total productivity of the process.

FIG. 3 represents a typical mass-spectrum of C₂F₄ products generated bythe procedure described above using the apparatus of FIG. 2. Thepressure of the CF₃H sample in cell 1 is 7.5 mbar, the time-delaybetween the end of the pre-excitation pulse 2 and the beginning of thedissociating CO₂ laser pulse 3 is 75 ns, and the CO₂ laser fluence isadjusted to 2.5 J/cm². The spectrum has been obtained after 10 minirradiation of the sample with the lasers operating at 10 Hz laserrepetition rate. The observed ratio of signals at masses 100-102corresponds to >99% ¹³C concentration in the C₂F₄ product. The estimateddissociation yield for each pair of laser pulses is 1.5-3% of all ¹³CF₃Hmolecules within the irradiated volume. These results represent a recordin ¹³C performance by MLIS.

The importance of collisional vibrational deactivation of CF₃H moleculespre-excited to the 3υ₁ level is illustrated by FIGS. 4 and 5.

FIG. 4 represents the pressure dependence of the concentration of ¹²C(triangles, left hand scale; experiment (a)) and ¹³C (squares, left handscale; experiment (b)) in the C₂F₄ dissociation product. In experiment(a), the wavelength of the first laser was tuned such that the amount ofpre-excited ¹²CF₃H at 10 mbar is about the same as the amount ofpre-excited ¹³CF₃H in experiment (b) at 10 mbar.

The experiments of FIG. 4 have been performed at zero time-delay betweenthe two laser pulses and with at CO₂ laser fluence of 7 J/cm². In thiscase, the time-lag Δt=about 35 ns, as explained below for FIG. 5. Allother parameters are as indicated above. One can see that the peakconcentration of ¹²C and ¹³C in the C₂F₄ occurs at different pressure,and this is a result of more rapid collisional vibrational deactivationof the pre-excited ¹²CF₃H as compared to ¹³CF₃H during the CO₂ laserpulse. The total number of the pre-excited molecules grows linearly withpressure, but competition with collisional deactivation reduces thenumber of pre-excited CF₃H that can be dissociated at a given fluence ofthe CO₂ laser. Consequently, the amount of C₂F₄ produced initiallyincreases with increasing pressure but then drops. The rate of this dropis different for different carbon isotopes.

FIG. 5 shows the concentration of ¹²C (triangles, left hand scale;experiment (a)) and ¹³C (squares, left hand scale; experiment (b)) inthe C₂F₄ dissociation product and the percentage of ¹³C in the C₂F₄dissociation product (circles, right-hand scale; experiment (b)) as afunction of time-delay between the overtone excitation laser pulse andthe CO₂ laser pulse. In experiment (a), the wavelength of the firstlaser was tuned such that about the same amount of ¹²CF₃H waspre-excited as the amount of ¹³CF₃H in experiment (b). Because thesemeasurements are made in two different experiments, the percentage of¹³C cannot simply be calculated from the concentrations of both isotopesshown in the Figure.

FIG. 5 illustrates the productivity for the two isotopes and thepercentage of ¹³C in the C₂F₄ product as a function of the time-delaybetween the two laser pulses at 5 mBar CF₃H pressure and CO₂ laserfluence of 3.5 J/cm⁻². It is clear that the percentage of ¹³C in C₂F₄grows with increasing delay up to about 200 ns—that is with increasingnumber of vibrationally deactivating collisions. One can see thatbecause of this deactivation, the total amount of produced C₂F₄ drops asa function of time delay, but it does so at different rates for the twodifferent isotopes of carbon.

Thus, both the final percentage of ¹³C in the C₂F₄ product and theproductivity of the ¹³C separation process can be controlled by theparameter P·Δt, where P is CF₃H pressure and Δt is a time between thepre-excitation by the first laser pulse and the dissociation during thesecond laser pulse. This implies a time-delay between the two laserpulses and an effective duration of the dissociating pulse (i.e., thetime the pulse is on before dissociation, which we have determined to beabout 35 ns for the CO₂ laser pulse shape used here). Production of C₂F₄highly enriched in ¹³C therefore requires this parameter, P·Δt, to belarge enough for near all vibrationally pre-excited ¹²CF₃H to becollisionally deactivated. For a quantitative estimate of the optimalparameter P·Δt, values for the vibrational deactivation constants fromthe 3υ₁ level have been determined experimentally to be about ¹³K₃=1.5μs mbar and ¹²K₃=4.5 μs mbar for ¹³CF₃H and ¹²CF₃H respectively.

Another aspect illustrated by this example is the relatively lowfluences of the pump and the dissociating radiation required for theprocess to be highly selective while retaining a reasonably high levelof productivity. This permits the use of collimated rather than focusedlaser beams, provided the pulse energies of two lasers are high enough.This allows the volume in which both laser energy fluences are in theoptimal range to be much larger than what can be achieved with focusedbeams.

The difference between collimated and focused beams is illustrated asfollows. Suppose the pre-excitation laser delivers 0.5 J pulse energy inan about 8 mm beam with divergence 2·10⁻³. This gives an energy fluenceof about 1 J/cm². This can be achieved, for example, by stimulated Ramanscattering of output of an alexandrite solid state laser in highpressure H₂. A dissociating beam of the same diameter and divergence andwith 2-3 J/cm² energy fluence can be produced by a TEA CO₂ laser. Thesetwo beams can be overlapped for a length of up to 3.3 meters before thefluences will drop to a half of their initial values because ofdivergence. This gives about a 0.25 liter irradiated volume where theprocess occurs. After each pass, the beams can be slightly recollimatedand sent again to the reactor to increase the active volume by severaltimes. This can be compared with a typical active volume of a few mm³achieved in experiments with focused laser beams. Thus, the low laserfluences required for the described process allows irradiation of largevolumes by collimated beams.

What is claimed is:
 1. A method of separating a desired isotope from astarting material comprising polyatomic molecules containing differentisotopes, by applying to said molecules in the gas phase, at apredetermined pressure P, infrared radiation of a first pulsed laser toexcite by a single photon an overtone transition of a light-atom stretchvibration of said molecules to produce vibrationally pre-excitedmolecules enriched in the desired isotope, and, infrared radiation of asecond pulsed laser of different frequency to induce a chemical reactionresulting in a molecule constituting a dissociation product, enriched inthe desired isotope, which can be separated from the starting material,wherein: the radiation of the first laser has a predetermined frequencyoptimized to select the desired isotope via a single-photon transition;the first laser pre-excites the polyatomic molecules to an excitedvibrational level that is high enough in energy to allow an increase inisotopic selectivity by collisions; the radiation of the second laserselectively induces dissociation of the vibrationally pre-excitedmolecules by infrared multiphoton excitation, at a time Δt after thepre-excitation by the first laser pulse; the pressure, P, and the time,Δt, between the pre-excitation by the first laser pulse and thedissociation during the second laser pulse have a pressure-time productP·Δt, the magnitude of this pressure-time product P·Δt beingsufficiently high to allow collisional vibrational deactivation of asubstantial amount of the vibrationally pre-excited molecules containingnon-desired isotope(s) before dissociation of the vibrationally excitedmolecules occurs while achieving significantly less collisionalvibrational deactivation of the pre-excited molecules containing thedesired isotope; whereby said collisions that occur in said time Δtbetween pre-excitation by the first laser pulse and the dissociationduring the second laser pulse enhance the isotopic selectivity of theisotopic separation method, such that said dissociation products ishighly enriched in the desired isotope.
 2. The method of claim 1 whereinthe polyatomic molecules of the starting material consist mostly of ¹²Cisotopes and further contain ¹³C isotopes, said polyatomic moleculescontaining C—H and C—F bonds, said method separating the ¹³C isotopesfrom the polyatomic molecules.
 3. The method of claim 2, wherein thepolyatomic molecules of the starting material are of the formula HCF₂X,where X is F, Cl, B or I.
 4. The method of claim 2, wherein thepolyatomic molecules of the starting material are trifluoromethane CHF₃and the predetermined frequency of the first laser is 8753±1 cm⁻¹ or8549±1 cm⁻¹ and the predetermined frequency of the second laser is inthe range 1020-1070 cm⁻¹.
 5. The method of claim 4, wherein the firstlaser produces at least 80% of said pre-excited molecules containing ¹³Cand at most 20% of said pre-excited molecules containing ¹²C and themagnitude of said pressure-time product P·Δt is sufficient to achievesaid dissociation products enriched to >95% in ¹³C isotopes for eachpair of the laser pulses.
 6. The method of claim 5, wherein the firstlaser produces about 90% of said pre-excited molecules containing ¹³Cand at most 10% of said pre-excited molecules containing ¹²C and themagnitude of said pressure-time product P·Δt is sufficient to achievesaid dissociation products enriched to >99% in ¹³C isotopes for eachpair of the laser pulses.
 7. The method of claim 2, wherein thepolyatomic molecules of the starting material are trifluoromethane HCF₃and the predetermined frequency of the first laser is 5936.5±1 cm⁻¹ or5681±1 cm⁻¹.
 8. The method of claim 2, wherein the polyatomic moleculesof the starting material are CF₂HCl and the predetermined frequency ofthe first laser is 5911±5 cm⁻¹ or 8693±2 cm⁻¹.
 9. The method of claim 2,wherein the polyatomic molecules of the starting materials aremonofluoromethane CH₃F.
 10. The method of claim 1, wherein thepolyatomic molecules of the starting material are selected from SiH₄,SiF₃H, SiCl₃H, GeH₄ and alcohols of the formula R—OH where R=CH₃, C₂H₅,C₃H₇ or C₄H₉.
 11. The method of claim 1 wherein the first and secondlasers are collimated or slightly diverging or slightly converging beamsof low fluence overlapping with one another over a substantial portionor all of their respective volumes containing the polyatomic molecules.